"Overnight" is defined as 9pm to 9am, and the lowest temperature in Melbourne in this period was 27.4 degrees – at 9am.

The previous warmest overnight December was 26.6 degrees.

Bureau of Meteorology senior forecaster Richard Carlyon said it was not the warmest minimum temperature, as that is measured from 9am the previous day to 9am on the current day. The mercury dropped to 24.4 degrees on Sunday morning, so that is recorded as the minimum.

State Emergency Services have been inundated with calls for assistance since midnight, with 187 people reporting fallen trees and traffic hazards, mainly in metropolitan areas. An SES spokesman advised residents to ensure any loose items around their homes have been secured.

The long-awaited cool change broke through soon after the record was broken, with winds of up to 90 km/h blowing across Port Phillip Bay and Bass Strait.

The Bureau issued a strong wind warning yesterday afternoon, which was upgraded to a gale warning early this morning.

Sunday night was also the hottest night for any month since January 1997, when the temperature was 28.8 degrees.

Today’s temperature is expected to peak at 31 degrees with a chance of rain while the forecast for Christmas Day remains unchanged, with expectations of morning showers and a dry afternoon, setting things up for a warm and dry start to the Boxing Day test, with an expected top temperature of 26 degrees.

Bureau of Meteorology duty forecaster Michael Efron said it wasn’t unusual to have several hot nights at this time of year. He said it was not an indicator that the rest of summer would be warmer-than-usual.

‘‘You have to look at them in isolation. Yesterday was also one of the warmest December days since 2010, but it’s nothing too unusual so far.’’

At Chadstone, retailers were hoping to break records of their own, as shoppers raced in for last-minute gifts and food.

Some 85,000 people took refuge from the heat and opted for the air conditioned comfort of the shopping centre between 6pm and 6am.

Centre manager Daniel Sutton said crowds were slightly up on previous years.

"We've had about 170,000 people through from 8am Sunday," he said.

Shops will stay open until 6pm Monday and will reopen for the Boxing Day sales on Wednesday morning.

On Sunday, Melbourne had its hottest day since New Year’s Eve 2010, with temperatures reaching 39.2 in some parts of the city.

Temperatures in the Melbourne CBD reached 38.3 shortly before 5pm, and remained above 34 at 7.25pm.

The state’s top temperature of 41.9 degrees was recorded at Walpeup in state’s north-west at 3.18pm.